How much is a used iPhone XR worth?

There’s no good way to answer that. It depends on model details and condition. Search eBay for and see what representative prices are. If you want to trade it in to Apple go to https://apple.com/iPhone, pick out a model, click Buy and chose to trade it. It will tell you what your phone is worth on a trade in. Or contact your carrier and ask what they will offer.

We can't tell you how much it's worth. What is its condition? Flawless? Good? OK? Broken?

Are you looking to buy one? Sell one?

You can check used sites to see what others are charging, like eBay, Offerup, Facebook Marketplace, etc.